CSS Selector for Child Theme

I am developing a WordPressMU site right now that is using a parent theme that I have developed along with a child theme that I would like to use for sub-sites. In the sub-sites, the only major change would be the header image that has a different city name on it. I would like to build a theme CSS selector into the admin area for the child theme. Any ideas on how to do this simply? Tutorial or sample code would be fantastic if there are any ideas out there.